Use apt-get to install towncrier

[johannaengland]

This has become necessary due to PEP 668.

Example where currently the towncrier check fails: https://github.com/Uninett/nav/actions/runs/11328705605/job/31502544146?pr=3140

The test workflow is save (for now) since it only runs on Python 3.9 and 3.10.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 60.47%. Comparing base (a0c30ea) to head (b4e58bd).

Additional details and impacted files

@@           Coverage Diff           @@
##           5.11.x    #3145   +/-   ##
=======================================
  Coverage   60.46%   60.47%           
=======================================
  Files         605      605           
  Lines       43809    43809           
  Branches       48       48           
=======================================
+ Hits        26491    26495    +4     
+ Misses      17306    17302    -4     
  Partials       12       12           

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[sonarcloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarCloud